The drive train in many ore grinding mills includes a ring gear and at least one pinion that is driven by a suitable known art motor. In such installations the gears are the weakest link in the drive chain and though designed for up to four times normal stresses (factor of safety of 400%) still in unusual circumstances are subjected to damaging mechanical stress conditions, for example, the motor can under certain fault conditions generate up to about ten to twelve times its rated torque. Similarly conditions to which the teeth of the gears are subjected, if the mill is run under resonance conditions for too long a period of time, will build up to exceed by a significant margin the maximum design four hundred percent factor of safety for the gears.
It is extremely important that the gearing even though it is the weakest link should not fail since the replacement cost of a gear set may be well over a million dollars and more important manufacturing lead time is likely to be in the order of at least 8 months and up to about 12 months which may require a long and costly shutdown period.
As above indicated these damaging overload conditions are not encountered frequently. The problem of mechanical resonant frequencies with constant speed drive mills has generally been dealt with during the design of the mill by adjusting motor shaft stiffness etc., before the equipment is built so that the operation is carried out remote from resonant torque amplification values. Obviously this requires some compromise between the various design options. When constant speed operation is contemplated this design technique has been satisfactory even though it is common to encounter at least one resonant condition during mill acceleration to operating speed. It is essential that the mill reach operating speed and pass through these resonant conditions without any permanent damage. Therefore, the mill is normally brought to speed by rapidly traversing the resonant frequency zones. These resonant speeds are passed through rapidly enough so that torsional oscillations do not have time to build up to a dangerous level and cause mill damage particularly gear damage.
In modern mills the use of variable operating speeds selected by the mill operator located in a remote control room, increases significantly the opportunity for the mill to operate right on or immediately adjacent a resonant frequency speed thereby increasing the requirements for gear protection.
Clutches are used in the drive chain of the mill for example to control the torque applied to the mill from two separate power sources to balance the torque so applied between the power sources. Canadian Pat. No. 934,679 issued Oct. 2, 1973 to Eastcott et al utilizes clutches in the drive chain to share the load by intermittently disengaging the clutches for preselected periods of time depending on the torque being transmitted. These clutches are also used during start up to bring the mill to speed by controlled clutch slippage.